In my particular case, I created my outline in AT3 and, in the process, set up AT3 custom fields before creating my Scrivener project. Doing it this way has the benefit of having AT3 set up the chapters and scenes (from the outline) and custom metadata fields in Scrivener for me. It’s not for everyone, but I thought I would mention it in case you haven’t yet created your Scrivener project. For me, it saved having to gain a deeper understanding of how Scrivener works before getting started on the drafting itself. (The deeper understanding flows from the process of drafting, but at least I didn’t have to spend hours and days learning Scrivener before getting started.)
Others here will be able to offer you more help than I can, but in case it’s helpful, here are my AT3 sync settings: